A fused cast refractory (which may be hereinafter referred simply to as a refractory) has been frequently used as a refractory for a glass melting furnace.
A fused cast refractory is a dense refractory that is markedly excellent in corrosion resistance to molten glass, and is produced in such a manner that raw materials including a major component, such as alumina, silica and zirconia, and a minor component, such as soda and boric acid, mixed in prescribed amounts are melted in an electric furnace, and the molten material is cast in a heat resistant mold and cooled in an annealing material, thereby solidifying the material in the shape of the mold.
Example of the fused cast refractory is a high zirconia fused cast refractory containing ZrO2 in an amount of 80% by weight or more.
The high zirconia fused cast refractory has an excellent corrosion resistance to any kind of molten glass due to the high content of ZrO2 and the dense structure thereof.
The high zirconia fused cast refractory also has a property that no reaction layer is formed at an interface to molten glass, which provides an excellent feature that defects including stones and cords may not be formed in the molten glass.
Accordingly, the high zirconia fused cast refractory is suitable for production of high quality glass.
The most part of the mineral structure of the high zirconia fused cast refractory is occupied by monoclinic zirconia crystals, and a small amount of a glass phase is filled in the grain boundaries of the zirconia crystals.
The characteristics of the high zirconia fused cast refractory are largely influenced by the kinds and the amounts of the components constituting the glass phase.
In general, the glass phase of the high zirconia fused cast refractory is constituted by oxides including Al2O3, SiO2, Na2O, B2O3 and P2O5.
The zirconia crystals undergo reversible transformation between a monoclinic and a tetragonal system associated with a rapid volume change around a temperature of from 1,000° C. (on cooling) to 1,150° C. (on heating).
The stress caused by the volume change associated with the transformation of the zirconia crystals is absorbed through flowage of the glass phase filled in the grain boundaries, and thereby a high zirconia fused cast refractory that is free of cracks in production and on heating may be produced in an industrial level (subject 1).
A glass melting furnace having the high zirconia fused cast refractory frequently uses burners as a heat source. In a glass melting furnace with burners, the burners are switched every several tens of minutes, and the temperature on the surface of the fused cast refractory varies by switching on and off the burners.
Consequently, the fused cast refractory, which is often used over several years, undergoes a considerably large number of heat cycles.
The high zirconia fused cast refractory may form in some cases zircon (ZrO2.SiO2) through reaction of silica (SiO2) which is a major component of the glass phase of the refractory, with zirconia (ZrO2) on receiving heating or heat cycles.
In this case, zircon crystals are formed in the glass phase, and thus the formation of zircon crystals may cause relative decrease of the glass phase. On progress of the decrease of the glass phase due to growth and increase of the zircon crystals, furthermore, it may be difficult to absorb the rapid volume change of the zirconia crystals around a temperature of from 1,000 to 1,150° C.
As a result, when the amount of the zircon crystals is increased beyond a certain level, the residual volume expansion rate of the refractory is increased extremely, which may cause formation of cracks due to deterioration in strength of the refractory structure, and finally pulverization in some cases.
Accordingly, there is a demand of a high zirconia fused cast refractory that is hard to form zircon crystals and is stable against heat cycles (subject 2).
Furthermore, even in a high zirconia fused cast refractory that is hard to form zircon crystals on heating and heat cycles to the refractory solely, zircon crystals may be formed easily through contact with molten glass in some cases.
In particular, zircon crystals are often liable to form on using the high zirconia fused cast refractory in a melting furnace for non-alkali glass, such as glass for a liquid crystal display (LCD) panel (which may be hereinafter referred to as liquid crystal glass).
The zircon crystals are formed due to the phenomenon that on melting the glass, the components of the molten glass and the glass phase of the high zirconia fused cast refractory are exchanged due to the difference in concentration of the components.
Specifically, the component that suppresses the formation of zircon crystals in the high zirconia fused cast refractory may be diffused into the molten glass, or a component that is liable to form zircon crystals may be migrated to the refractory from the molten glass. It is considered that any one or both of the phenomena may occur to facilitate the formation of zircon crystals in the high zirconia fused cast refractory.
In the state where zircon crystals are formed in the high zirconia fused cast refractory used in a glass melting furnace, the amount of the glass phase is decreased thereof, the rapid volume change of the zirconia crystals around a temperature of from 1,000 to 1,150° C. may not be absorbed as described above.
Consequently, on receiving heat cycles due to heating on operation and variety in temperature on operation, the residual volume expansion rate of the refractory may be increased extremely, and the strength of the structure may be decreased, thereby easily causing cracks in the refractory. The refractory may be corroded selectively from the cracked portions, and with the progress of the corrosion, small pieces of the refractory may fall down to the molten glass to deteriorate the quality of the glass.
In the case where a high zirconia fused cast refractory that is hard to form zircon crystals through contact with molten glass is used as a furnace material, the material may be stable without the formation of zircon crystals on receiving heat cycles due to heating on operation of a glass melting furnace and variety in temperature on operation, and may be free of cracks. Furthermore, on cooling for stopping the production in the glass melting furnace, formation of new cracks and growth of cracks having been formed may be prevented.
As a result, the high zirconia fused cast refractory may be reused without replacement for the next operation after stopping operation.
As described above, there is a demand of a high zirconia fused cast refractory that is hard to form zircon crystals even under conditions where the refractory is in contact with molten glass (subject 3).
The high zirconia fused cast refractory that is hard to form zircon crystals has been investigated.
PTL 1 (JP-A-63-285173) describes a high electric resistance high zirconia fused cast refractory that contains from 90 to 98% of ZrO2 and 1% or less of Al2O3, contains substantially no Li2O, Na2O, CaO, CuO or MgO, and contains from 0.5 to 1.5% of B2O3, or from 0.5 to 1.5% of B2O3 with 1.5% by weight or less of at least one of K2O, SrO, BaO, Rb2O and Cs2O.
In PTL 1, however, the glass phase of the high zirconia fused cast refractory contains a large amount of B2O3, which facilitates formation of zircon crystals, and thus the residual volume expansion rate after a heat cycle test may be large, which provides a defect of formation of zircon with the refractory solely. Furthermore, CaO for stabilizing the glass phase through control of the viscosity thereof is not contained, and thus cracks may be formed in a one-sided heating test due to failure of absorbing the stress formed in the production.
PTL 2 (JP-A-8-48573) proposes a high zirconia fused cast refractory that has high electric resistance and heat cycle stability and contains from 85 to 96% by weight of ZrO2, from 3 to 8% by weight of SiO2, from 0.1 to 2% by weight of Al2O3, the content of B2O3 of from 0.05 to 3% by weight, the content of Na2O of 0.05% by weight or more, the content of Na2O and K2O of from 0.05 to 0.6% by weight, and the content of BaO, SrO and MgO of from 0.05 to 3% by weight.
In PTL 2, however, CaO for stabilizing the glass phase through control of the viscosity thereof is not contained, but a large amount of MgO, which markedly facilitates formation of zircon crystals under conditions where the refractory is in contact with molten glass, and thus it is insufficient to suppress the formation of zircon crystals under conditions where the refractory is in contact with molten glass.
PTL 3 (JP-A-9-2870) proposes a high zirconia fused cast refractory that suffers less cracks in production and less cracks on heat cycles and contains from 89 to 96% by weight of ZrO2, from 2.5 to 8.5% by weight of SiO2, from 0.2 to 1.5% by weight of Al2O3, less than 0.5% by weight of P2O5, less than 1.2% by weight of B2O3, less than 0.3% by weight of CuO, more than 0.01 and less than 1.7% by weight of P2O5+B2O3, from 0.05 to 1.0% by weight of Na2O+K2O, from 0.01 to 0.5% by weight of BaO, less than 0.5% by weight of SnO2, and 0.3% by weight or less of Fe2O3+TiO2.
PTL 3 states that the addition of Na2O, K2O and BaO suppresses the formation of zircon crystals even though the components that facilitate the formation of zircon crystals, such as P2O5 and B2O3, are contained.
However, the amount of BaO added for suppressing the formation of zircon crystals is small, but P2O5, which markedly facilitates the formation of zircon crystals, is contained, and the formation of zircon crystals may not be suppressed under conditions where the refractory is in contact with molten glass.
Furthermore, SnO2 is not an essential component, and the effect of the addition of SnO2 is not known since the effect of SnO2 on the cracks in production and the cracks after heat cycles is not described.
PTL 4 (JP-A-2008-7358) proposes a high zirconia fused cast refractory that has excellent heat cycle stability and high electric resistance and contains from 87 to 96% by weight of ZrO2, 0.1 or more and less than 0.8% by weight of Al2O3, from 3 to 10% by weight of SiO2, less than 0.05% by weight of Na2O, from 0.01 to 0.2% by weight of K2O, from 0.1 to 1.0% by weight of B2O3, from 0.1 to 0.5% by weight of BaO, less than 0.05% by weight of SrO, from 0.01 to 0.15% by weight of CaO, from 0.05 to 0.4% by weight of Y2O3, less than 0.1% by weight of MgO, 0.3% by weight or less of Fe2O3+TiO2, and less than 0.01% by weight of each of P2O5 and CuO.
However, the amount of BaO and SrO added, which are components suppressing the formation of zircon crystals, is small, and thus the formation of zircon crystals is insufficiently suppressed under conditions where the refractory is in contact with molten glass.
PTL 5 (WO 2012/046785A1) proposes a high zirconia fused cast refractory that is hard to form zircon crystals and contains from 86 to 96% by weight of ZrO2, from 2.5 to 8.5% by weight of SiO2, from 0.4 to 3% by weight of Al2O3, from 0.4 to 1.8% by weight of K2O, 0.04% by weight or less of B2O3, 0.04% by weight or less of P2O5, 3.8% by weight or less of Cs2O, and substantially no Na2O.
However, the content of B2O3 and P2O5, which prevent cracks in production, is very small, but an alkali metal oxide, such as K2O and Cs2O, is contained in a large amount, and thus it is insufficient for producing a large-scale product suffering less cracks in production and on heating.
Furthermore, Cs2O is a considerably expensive material to provide a problem in industrial production.
PTL 6 (WO 2012/046786A1) proposes a high zirconia fused cast refractory that is hard to form zircon crystals and contains, as essential components, from 85 to 95% by weight of ZrO2, 2.5% by weight or more of SiO2, 0.04% by weight or less of Na2O, 0.04% by weight or less of B2O3, and 0.04% by weight or less of P2O5, in which at least one of K2O and Cs2O is contained and the following expressions (1) and (2) are satisfied.0.2≦0.638×C(K2O)+0.213×C(Cs2O)+0.580×C(SrO)/C(SiO2)≦0.40  (1)0.10≦0.580×C(SrO)/C(SiO2)  (2)wherein C(X) represents the content (% by weight) of the component X.
PTL 6 states that the high zirconia fused cast refractory proposed suffers less cracks in production and is hard to form zircon crystals even in contact with molten glass by specifying the molar concentration ratios of K2O, Cs2O and SrO with respect to SiO2 in the expressions (1) and (2).
In PTL 6, however, the content of B2O3 and P2O5, which prevent cracks in production, is very small, but an alkali metal oxide, such as K2O and Cs2O, is contained in a large amount, and thus it is insufficient for producing a large-scale product suffering less cracks in production and on heating.
Furthermore, Cs2O is a considerably expensive material to provide a problem in industrial production.